1. Field of the Invention
This invention relates to a catalyst for conversion of a hydrocarbonaceous feed, for example, a zeolite catalyst for aromatization of alkanes to aromatics, e.g., a MFI-type zeolite catalyst for the aromatization of alkanes having two to six carbon atoms per molecule to aromatics, such as benzene, toluene and xylenes.
2. Description of the Prior Art
Zeolite is a crystalline hydrated aluminosilicate that may contain other metals in the framework of the zeolite crystal or deposited, exchanged or impregnated onto the zeolite. A method for preparing a zeolite comprises (a) preparing an aqueous mixture of silicon oxide and sources of oxides of aluminum; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of zeolite form. Zeolite by itself is known as an operative catalyst for many hydrocarbon conversion reactions but selectivity to a particular product may be low. Much zeolite research has focused on the zeolite framework containing elements other than silicon and aluminum and on depositing particular metals on the surface of the zeolite.
U.S. Pat. No. 5,672,796 discloses a catalyst and process for aromatizing one or more C3 to C6 saturated hydrocarbon with a partially sulfided Pt/Re loaded crystalline aluminosilicate molecular sieve of MFI structure and a Si/Al ratio between about 40 to about 600 to produce benzene, toluene, xylenes and ethane or ethane and propane with low methane content.
U.S. Pat. No. 4,766,265 discloses a process for the conversion of ethane to liquid aromatic hydrocarbons with a gallium modified molecular sieve catalyst promoted with rhenium and one of nickel, palladium, platinum rhodium and iridium. The molar ratio of silicon to M where M is aluminum, boron or gallium is in the range of about 10:1 to about 100:1, preferably ranging from about 20:1 to about 50:1.
U.S. Pat. No. 4,835,336 discloses a process for converting a C6-C12 paraffinic hydrocarbon feed to aromatics with a noble metal/low acidity medium pore size zeolite catalyst which has been presulfided. Adding a sulfur compound such as H2S, SO2 or an organic sulfur compound to the feed is an alternative to presulfiding. The silica to alumina mole ratio was disclosed as at least 12, preferably at least about 30, and could be 1600 and above. The example was a ZSM-5 having a silica to alumina ratio of 26,000:1.
U.S. Pat. No. 4,861,932 discloses a process for converting a C2-C12 paraffinic hydrocarbon feed to aromatics by contacting the feed with a noble metal/low acidity medium pore size zeolite catalyst in a first conversion zone then passing the resulting hydrocarbon mixture through a second conversion zone and contacting it with a medium pore size acidic zeolite catalyst. The silica to alumina mole ratio was disclosed as at least 12, preferably at least about 30, and could be 1600 and above. The example was a Pt/ZSM-5 having a starting silica to alumina ratio of 26,000:1 in the first conversion zone and a Ga/Ti-ZSM-5 having a starting silica to alumina ratio of 70:1 in the second conversion zone.
U.S. Pat. No. 4,891,463 discloses a process for converting C2 to C12 aliphatic hydrocarbons to aromatics with a crystalline zeolite catalyst containing gallium. The final catalyst will usually have a silica/alumina ratio of at least about 12. Preferably the silica/alumina ratio is about 500 to 26,000. The catalyst may contain an added metal which is gallium or any of the various suitable metals in Groups I through VIII of the Periodic Table including zinc, platinum, rhenium, cobalt, titanium, tellurium, sodium, nickel, chromium, aluminum, copper, palladium, calcium and rare earth metals.
It would be advantageous to have a zeolite-type catalyst which maintained relatively constant selectivity for conversion of lower alkanes, such as alkanes having two to six carbon atoms per molecule, to aromatics, such as benzene, toluene and xylene, over a period of time on stream.